Ultraviolet light sources (UV-A, B, C) are not only widely utilized as light sources for exposure, but also expected to be used for environmental/medical fields utilizing the strong antiseptic actions or photochemical reactions, and further to be extensively used for decomposition of environmental pollutants, water quality management, and the like.
At the present time, mercury lamps (with a luminescence ultraviolet wavelength of 254 nm) are mainly used as ultraviolet light sources. In the case of a mercury lamp, its electron source is of a filament type, the light source by electronic excitation naturally becomes a large vacuum-tube device such as a fluorescent lamp. Therefore, the current ultraviolet light sources have problems such as a risk of operating loss due to short lifetime/vacuum tube rupture or difficulty in downsizing of device chips.
Further, a countermeasure for the RoHS directive is also important. This is officially referred to as the “Directive on the Restriction of the Use of Certain Hazardous Substances in Electrical and Electronic Equipment,” that has taken effect from July 2006 in all European Union member states. With respect to electrical and electronic equipment, the usage rates of six hazardous substances which are lead, mercury, cadmium, hexavalent chromium, polybrominated biphenyls (PBB), and polybrominated diphenyl ether (PBDE) are restricted. Here, mercury is restricted to be 1,000 ppm or less, and therefore, the development of mercury-free light sources in place of mercury lamps is urgently needed.
Against this backdrop, nitride semiconductor light-emitting diodes are recently actively researched as mercury-free light sources in place of mercury lamps.
However, as matters stand, high-intensity luminescence cannot be achieved at a short-wavelength of less than 365 nm determined on the basis of a bandgap of gallium nitride. The reason for this difficulty in achieving high-intensity luminescence is that, although it is necessary to make a structure of a light-emitting diode in which a material of a luminescent active layer is sandwiched by materials with a bandgap higher than that of the luminescent active layer, in order to obtain luminescence out of a deep ultraviolet region, containment of carriers is insufficient even when the luminescent active layer is sandwiched by aluminum nitride with a maximum bandgap, and its luminous efficiency is extremely decreased. For example, with respect to a light-emitting diode using an aluminum nitride crystal as a semiconductor, luminescence of an ultraviolet light at a wavelength of 210 nm, and a short wavelength, has been reported (Non-Patent Document 1). Meanwhile, its luminescent output and external quantum efficiency are low, which results in a situation where the practical use thereof is difficult.
Further, on the other hand, an ultraviolet-emitting electroluminescence element (EL element) emitting light in an ultraviolet wavelength region is known (for example, refer to Patent Document 1). Such an EL element is configured to have a double insulating layer structure in which a light-emitting film is sandwiched by two-layered dielectric insulating films, to stably emit light. In detail, the ultraviolet-emitting electroluminescence element has a structure on a transparent substrate such as glass in which a transparent conducting film formed of ITO (Indium Tin Oxide) or the like, a lower insulating film formed of SiO2 or the like, a light-emitting film in which luminescence center elements are added into a host material, an upper insulating film formed in the same way as the lower insulating film, and a back surface conducting film formed of metal are laminated in series. With respect to an EL element, not only a luminous phenomenon in a visible light wavelength region, but also a luminous phenomenon in an ultraviolet wavelength region is known from long ago, and the luminescence in an ultraviolet wavelength region has been utilized as excitation energy for a phosphor, to perform a wavelength conversion of the luminescence into a visible light region (for example, refer to Patent Document 2).
As described above, a variety of attempts with respect to EL elements have been made. However, EL elements having sufficient performances for practical uses have not yet been realized, that brings about a situation where it is difficult to utilize the EL elements as key devices by applying a luminous phenomenon in an ultraviolet wavelength region to actual products.
[Patent Document 1] JP-A-2000-173775
[Patent Document 2] JP-A-Hei-11-195488
[Non-Patent Document 1] NTT Develops an Ultra-Violet LED Using Aluminum Nitride with an Extremely Short Wavelength of 210 nm (NIKKEI ELECTRONICS Jun. 19, 2006, P. 30, 31), Nature 441, 325(2006).